1. Field of the Invention
The present invention relates to a toner for developing latent electrostatic images for use in the fields of electrophotography, electrostatic recording and electrostatic printing.
2. Discussion of Background
Generally, in accordance with various kinds of method, latent electrostatic images are formed on a photoconductor using a photoconductive material contained in the photoconductor in the electrophotographic process. The latent electrostatic images thus formed on the photoconductor are developed into visible images with a toner. The visible toner images thus formed are transferred onto an image-receiving material such as a sheet of paper when necessary, and fixed thereon by heating or by applying a solvent vapor, so that the making of a print can be attained. For developing the latent electrostatic images formed on the photoconductor into visible toner images, various developing procedures such as magnetic brush development, cascade development, and powder cloud development are conventionally known. In any development procedure, it is an important step to fix visible toner images onto the image-receiving material.
In the case where the toner images are fixed onto the image-receiving material by using a heat-application roller, which is the most widely-used image fixing means, the heat-application roller is in contact with a toner in a fused state at the image fixing step. As a result, the fused toner partially sticks to the surface of the heat-application roller and the image-receiving material subsequently sent to the heat-application roller is stained with the fused toner, namely, a so-called off-set phenomenon occurs.
There is proposed in Japanese Laid-Open Patent Application 54-114245 a toner capable of attaining image fixing performance at a low temperature, and at the same time, preventing the off-set phenomenon. This toner comprises a low-molecular-weight polyester resin or epoxy resin which can be fused at low temperatures, in an amount of 50 to 95 parts by weight, and a giant-molecular-weight vinyl resin with a weight-average molecular weight of 500,000 or more. However, since there is a big difference in melting viscosity between the aforementioned polyester resin or epoxy resin and vinyl resin, they cannot be highly dispersed even when kneaded under the application of heat thereto with a strong shearing force applied thereto for a long period of time in the course of preparing the toner.
In general, in the case where resins having poor compatibility with each other are mixed together, a continuous phase and a disperse phase are generated in the obtained mixture as described in Plastic, 13, No. 9, 1P (1962). The dispersion properties of the resin mixture can be improved by making each of the finely-divided disperse phases dispersed in the continuous phase small. It is considered that the closer the melting viscosities of resins to be mixed together, the better the dispersion properties of the obtained resin mixture when the resins are kneaded under the application of heat thereto. When the toner obtained in the previously mentioned Japanese Laid-Open Patent Application 54-114245 is observed with a transmission-type electron microscope, a disperse phase comprising styrene--butadiene resin is present in the form of fairly large particles in a continuous phase comprising polyester, and further, carbon black particles serving as a coloring agent are not uniformly dispersed in the toner particle. In addition, when this toner is mixed with a carrier of iron powder to prepare a two-component type developer, the distribution of the charge quantity of the toner is considerably widened, and fogging is caused after copying is repeatedly carried out.
Moreover, in the case where a releasing agent is contained in a toner to prevent the off-set phenomenon, the compatibility of the releasing agent with the binder resins generally used in the toner is considerably poor since the molecular weight of the releasing agent is extremely low. Consequently, the releasing agent is dispersed in the form of large particles in a toner particle. Thereafter, the releasing agent bleeds out toward the surface of the toner particle with the application of heat thereto when the toner particles are stirred and come into collision in a development unit. Thus, the releasing agent on the surface of the toner particle adheres to the surface of a carrier particle or a charge-application member provided in the development unit, such as a sleeve. Due to the adhesion of the toner to the carrier, namely, a so-called spent phenomenon, the durability of the obtained developer remarkably deteriorates.
As previously mentioned, there has not been proposed a toner with high durability, capable of carrying out image fixing performance at a low temperature without causing the off-set phenomenon and forming a toner image which does not adhere to a vinyl chloride product.